New Study Casts Doubt on Reliability of Glycemic Index

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The glycemic index, the often-touted tool of dieters and diabetics, might not be as effective as it once seemed. A new study from Tufts University found inconsistent results in its ability to predict how foods would affect individuals' blood sugar levels.

Hayley Sugg

September 08, 2016

With food divided into three categories, low GI, medium GI, and high GI, the glycemic index is (in theory) supposed to help individuals predict how different foods will affect their blood sugar, and ultimately their health. It's a commonly used method among people with diabetes, and it is also used in popular diets like Nutrisystem. But a study by scientists at the Jean Mayer USDA Human Nutrition Research Center on Aging has found that opting for whole foods may be a better way to choose the healthiest options, not a ranking on the glycemic index.

The study, conducted with 63 individuals over 12 weeks, tested where each person's blood sugar was after consuming either a piece of white bread (as a simple carbohydrate) or a glucose drink (as a control measure). According to the glycemic index, the white bread is high on the index and therefore should cause a spike in blood sugar, but the study found that it affected each individual differently. Roughly a third of the participants' blood sugar reacted like it was a low glycemic food (with little to no effect on their blood sugar levels). Another third reacted as if it were a medium glycemic food, and the last third had a spike in their blood sugar that indicated it was a high glycemic food. There were also varying results for each individual depending on which day they participated.

"If someone eats the same amount of the same food three times, their blood glucose response should be similar each time, but that was not observed in our study," said study author Nirupa Matthan, Ph.D, to Science Daily, "A food that is low glycemic index for you one time you eat it could be high the next time, and it may have no impact on blood sugar for me."

Alice H. Lichtenstein, D.Sc., a senior study author and scientist, recommended that instead of following the glycemic index, it's best to choose a diet full of fruits, vegetables, whole grains, low-fat dairy, legumes, fish, and lean meats.